Freed Italian journalist Giuliana Sgrena has been hurt by US troops firing at a car taking her to Baghdad airport soon after her release.

An Italian secret service agent was killed in the shooting at a checkpoint.

Italian Prime Minister Silvio Berlusconi summoned the US ambassador demanding an explanation for what he called "such a serious incident".

The US military in Baghdad confirmed that forces shot at a vehicle and said an investigation had been launched.

"About 2100 [1800 GMT], a patrol in western Baghdad observed the vehicle speeding towards their checkpoint and attempted to warn the driver to stop by hand and arm signals, flashing white lights, and firing warning shots in front of the car," it said in a statement.

"When the driver didn't stop, the soldiers shot into the engine block, which stopped the vehicle, killing one and wounding two others."

'Dumbfounded'

Mr Berlusconi, a staunch supporter of the US-led invasion of Iraq, said there were "disquieting questions" that needed to be answered.

This was a joyful moment... which has been transformed into profound pain